Conventional molding techniques used to produce a desired part involve conducting heat to the mold body such that the surface temperatures of the mold cavity within the mold body allows for adequate flow of polymer to fill the cavity. It has become increasingly desirable to cycle the mold to a higher temperature to improve the surface or cosmetic quality of the part in addition to the physical properties of the part. However, temperature cycling to heat and cool the mold has been found to lengthen the cycle time due to the time required to supply and remove the thermal energy needed to raise and lower the temperature of the mold body.
Induction heating is one approach which has been used to achieve faster heating of the mold. Previous methods of induction heating used to mold or form polymer employ a conductive coil which surrounds the external surfaces of the mold. In another example, an induction coil that is inserted between the mold halves which heats the mold while the mold is open and then is retracted before closing the mold to produce the part. Other previous methods have employed inductive heating using conductive polymers which include magnetic reinforcements, also known as susceptors, dispersed within the polymer matrix. The induction heating coils heat the conductive polymer matrix disposed between two non-magnetic mold surfaces. The improved cycle times of induction heating in some cases, have justified the use of plastic as a replacement for larger structural parts such as metal body panels in automobiles, for example.
Other techniques which have been used to achieve rapid heating of the mold include infrared heating, resistive heating, laser heating and microwave heating, for example. These heating techniques allow for directed heating of specific regions of the mold to achieve higher temperatures than the bulk mold. These regions of the mold are typically those which are come into contact with and/or are closer to the polymer part surface. Hence less energy is required to achieve the mold surface temperatures required for good cosmetic quality/aesthetics. Also, as the rate of the heating is faster, it is possible to produce a polymer part in a much faster cycle time. The molding cycle times for producing polymer parts are not only improved by the above rapid heating techniques, but cycle time can also be reduced by cooling the mold prior to ejection of the part. That is, once the polymer is heated to the desirable temperature for forming the part, a cooling medium can be passed through the mold body to cool down the mold to a predetermined release or ejection temperature.
Nevertheless, although these heating methods which may quickly heat the mold surface, can still result in cycle times which are prohibitive for applications requiring large parts and/or high part volumes.